This invention relates to corrosion inhibiting compositions for aqueous systems comprising (a) polyaspartic acid or salts thereof, and (b) a water soluble phosphonated oligomer. The compositions are effective corrosion inhibitors, but are also advantageous from an environmental standpoint because they are non-toxic, contain no heavy metals, and have little or no phosphorus. They also have significant calcium/hardness tolerance, are chlorine/bromine stable, and are effective corrosion inhibitors.
Water used in industrial cooling or mining systems comes from rivers, lakes, ponds or from underground reservoirs. Such water contains dissolved inorganic salts. When this water circulates through the heat exchangers and cooling towers in a cooling system, a portion of the water is lost due to the evaporation. This increases the concentration of inorganic salts in the system. If the solubility of these salts in water is exceeded, precipitation will take place.
As the salts precipitate on the internal surface of a cooling system, they form scale or deposits. The scale inhibits effective heat transfer, restricts the flow of the water, and promotes the development of underdeposit corrosion. Consequently, it is necessary to remove the scale by cleaning. Such cleaning is expensive because equipment must be shutdown, labor costs are incurred, and production is delayed. In view of these problems, preventing scale formation is preferred to scale removal.
Scale formation can be inhibited by adding a sequestering or chelating compound to the water treatment system. The amount of a chelating/sequestering compound required is a stoichiometric amount based upon the amount of calcium and magnesium cations in the aqueous system cleaned. This method of the scale inhibition is expensive and not customarily used.
More than 50 years ago it was discovered that certain compounds performed as highly efficient scale inhibitors. Such compounds are used in significantly lower than stoichiometric amounts and are known as xe2x80x9cthreshold inhibitorsxe2x80x9d. Examples of threshold inhibitors are phosphonates and water soluble acrylic/maleic/sulfonic polymers or copolymers. Corrosion inhibitors, such as phosphonates, inorganic phosphates, azoles, zinc, and molybdate, are often used with scale inhibitors.
In addition to effective performance, water treatment chemicals must be environmentally acceptable. Environmental regulations prohibit the use of such corrosion inhibitors as chromates and restrictions are now prevalent for the use of all heavy metals. The trend is also toward water treatment chemicals that are non-toxic, have little or no phosphorus, have high calcium/hardness tolerance, are chlorine/bromine stable, and at the same time have high scale and corrosion efficacy. Because of these requirements, the cost of water has increased, causing higher reuse/higher cooling cycles which results in cooling waters with high hardness and alkalinity contents.
U.S. Pat. No. 5,523,023 relates to compositions comprising polyaspartic acid and phosphonobutane tricarboxylic acid which are used for alkaline cleaners. U.S. Pat. No. 5,386,038 discloses a water soluble mixture of phosphonated oligomers having the general formula:
H[CHRCHR]nxe2x80x94PO3M2
wherein at least one R group in each unit is a COOM, CH2OH, phosphono sulphono, sulphato, or phosphono group and the other R group which may be the same as, or different from, the first R group, is hydrogen or a COOM, hydroxyl, phosphono sulphono, sulphato, C1-7 alkyl, C1-7 alkenyl group or a carboxylate, phosphono, sulphono, sulphato, and/or hydroxyl C1-7 alkyl, C1-7 alkenyl group, and each M is a cation such that the phosphonated oligomer is water soluble and n is 1 to 6, preferably  greater than 1 and  less than 6. These compositions inhibit scale formation and/or the corrosion of metal exposed to aqueous systems.
This invention relates to corrosion inhibitor compositions for aqueous systems such as cooling waters, mining waters and geothermal waters having high levels of hardness and alkalinity. The compositions comprise:
(a) a water soluble polyaspartic acid or salt thereof; and
(b) a water soluble phosphonated oligomer having the general formula (I):
H[CHRCHR]nxe2x80x94PO3M2
wherein at least one R group in each unit is a COOM, CH2OH, sulphono, or phosphono group and the other R group which may be the same as, or different from, the first R group, is hydrogen or a COOM, hydroxyl, phosphono, sulphono, sulphato, C1-7 alkyl, C1-7 alkenyl group or a carboxylate, phosphono, sulphono, sulphato, and/or hydroxyl substituted C1-7 alkyl or C1-7 alkenyl group, and each M is a cation such that the phosphonated oligomer is water soluble and n is 1 to 6, preferably  greater than 1 and  less than 6.
The corrosion inhibiting compositions are synergistic because they inhibit corrosion to a greater extent than was expected in view of the corrosion inhibition activity of the individual components.
Component (a) of the corrosion inhibitor composition is a water soluble polyaspartic acid. for purposes of this invention, the term xe2x80x9cpolyaspartic acidxe2x80x9d shall be construed to include salts and derivatives of polyaspartic acid. Polyaspartic acid, salts thereof, and derivatives of polyaspartic acid are well known and are described in U.S. Pat. No. 5,523,023 which is hereby incorporated by reference. Preferably used is polyaspartic acid having a molecular weight, according to gel-permeation chromatographic analysis, of from 500 to 10,000, preferably 1,000 to 5,000, most preferably 2,000 to 4,000. The polyaspartic acid is preferably used as a salt, in particular as a sodium salt or potassium salt. Whether polyaspartic acid is used in the form of an acid or a salt depends upon the pH of the aqueous system treated. Preferably the salts of polyaspartic acid are sodium salts. Derivatives of polyaspartic acid, for example anhydrides of polyaspartic acid, which can convert into polyaspartic acid as a result of hydrolysis under use conditions, also can be used.
Component (b) of the corrosion inhibitor composition is a water soluble phosphonocarboxylic oligomer salt, preferably sodium salt, typically found as a mixture of oligomers. These oligomers are described in U.S. Pat. No. 5,386,038 which hereby is incorporated by reference. The general structural formula (I) for these water soluble phosphonocarboxylic oligomer salts is set forth previously under the Summary of the Invention. Preferably used as the water soluble phosphonocarboxylic oligomer salts are salts having the following specific version of the above general structual formula:
H[CH(CO2Na)xe2x80x94CH(CO2Na)]nPO3Na2
where xe2x80x9cnxe2x80x9d less than 5. Specific examples of water soluble phosphonocarboxylic oligomer salts include an aqueous mixture of phosphonocarboxylic acid oligomeric salts known as BRICORR 288, manufactured by AlbrightandWilson Inc. The weight ratio of phosphonated oligomer to polyaspartic acid to is from 8:1 to 1:12, preferably 4:1 to 1:9, more preferably from 1:4 to 1:9.
For some applications it is preferable to add a water soluble copolymer to the scale inhibiting composition, for instance phosphinocarboxylic polymer, maleic acid or maleic anhydride polymer, acrylic polymer, methacrylic polymer and their copolymers with sulfonic and/or phosphino functionalities, preferably acrylic/sulfonic copolymers or acrylic/maleic copolymers.
Other optional components include phosphonobutane tricarboxylic acid, tolyltriazole, orthophosphate, polyphosphates, hydroxyethylidene diphosphonic acid, amino tri(methylene phosphonic acid).
The corrosion inhibiting compositions are used at the minimum dosage of 0.1 ppm, but preferably in a dosage of 5.0 to 500.0 ppm, most preferably 10.0 to 200.0 ppm.
The following are the abbreviations used in the examples: